The present invention relates to a method and an arrangement for varying a pressure generated by a low-pressure pump and applied to a high-pressure pump. The low-pressure pump and the high-pressure pump pump fuel for an internal combustion engine.
Especially in modern direct-injecting internal combustion engines, pump arrangements are utilized to supply the engines with fuel and these pump arrangements include a low-pressure pump, the so-called presupply pump, and a high-pressure pump, the so-called primary supply pump. The low-pressure pump can be configured to be controlled as to requirements and then always pumps as much fuel as just needed by the high-pressure pump.
At specific operating points of the engine, a vaporization of fuel can occur in the high-pressure pump. The vapor formation in the high-pressure pump is facilitated by high temperatures in the high-pressure pump and by a low prepressure at which the fuel is applied to the high-pressure pump. When vapor is formed in the high-pressure pump, high pressure cannot be generated therein and the engine is only supplied inadequately with fuel which has negative effects on the operability of the engine.
From the state of the art, it is known to increase the prepressure with which the fuel is applied to the high-pressure pump in order to avoid the formation of vapor in the high-pressure pump. When utilizing a requirement-controlled low-pressure pump, the desired value of the fuel prepressure is variable and can be selected so high that under no circumstances a vapor formation occurs in the high-pressure pump. With increasing prepressure, the pumping capacity of the low-pressure pump, however, drops by approximately 20 liters per hour per 1 bar of pressure increase. At specific operating points of the engine, for example, at full-load operation, when the engine has a high requirement for fuel and simultaneously the fuel is applied with a high prepressure to the high-pressure pump, the low-pressure pump is greatly loaded and can, under circumstances, be pushed to its pumping limit which, in turn, has negative effects on the operability of the engine.
In an internal combustion engine, which is supplied with fuel by a low-pressure pump and a high-pressure pump, it is the task of the present invention, on the one hand, to reliably prevent a formation of vapor in the high-pressure pump and, on the other hand, to ensure the operability of the engine especially its supply of fuel at all operating points.
To solve this task, the invention proceeds from the method of the kind mentioned initially herein and suggests that:
the actual temperature of the fuel in the high-pressure pump is determined;
in dependence upon the fuel temperature, a prepressure as low as possible is determined at which a vaporization of the fuel in the high-pressure pump is reliably avoided; and,
the low-pressure pump is so controlled (open loop and/or closed loop) that it generates the determined prepressure.
According to the present invention, the prepressure is therefore controlled (open loop and/or closed loop) in dependence upon the actual temperature of the fuel in the high-pressure pump. This affords the advantage that a vapor formation in the high-pressure pump is in each case reliably avoided. The prepressure is in each case only selected so high in dependence upon the determined fuel temperature that a vaporization of the fuel in the high-pressure pump is reliably avoided. This affords the advantage that the prepressure in no operating point of the engine (for example, for reasons of safety or other reasons) has too high a value and the low-pressure pump is thereby unnecessarily loaded. This leads to a longer service life of the low-pressure pump. Furthermore, a low-pressure pump requires less energy for a reduced prepressure. A low-pressure pump, which is configured as an electrical fuel pump has a lower power consumption. Finally, a reduced tank heating and lower permeation losses result because of the reduced prepressure.
According to a preferred embodiment of the present invention, it is suggested that the fuel temperature be estimated based on a physical model of the high-pressure pump in dependence upon the temperature of the high-pressure pump and specific condition variables of the engine.
According to an advantageous embodiment of the present invention, it is suggested that the actual throughput of fuel in the engine is determined and the fuel temperature in the high-pressure pump is determined while considering the fuel throughput. The more fuel the engine takes up from the high-pressure loop, the more cool fuel from the tank can be supplied to the high-pressure loop via the low-pressure pump. The supplied cool fuel effects a lowering of the fuel temperature in the high-pressure pump and thereby counters a formation of vapor in the high-pressure pump. Accordingly, with a high throughput of fuel into the engine, the prepressure, at which the fuel is applied to the high-pressure pump, can be correspondingly lowered.
According to an advantageous embodiment of the present invention, it is suggested that the temperature of the high-pressure pump be estimated based on a physical model of the high-pressure pump in dependence upon specific condition variables of the engine.
According to a preferred embodiment of the present invention, it is suggested that the prepressure be determined based on a fuel vapor pressure characteristic line from which a value of the prepressure is taken to which a safety-reserve pressure is added. This value of the prepressure corresponds to the fuel temperature. The fuel vapor pressure characteristic line is a function of the prepressure in dependence upon the fuel temperature in the high-pressure pump. From the high-pressure vapor pressure characteristic line, the corresponding value of the prepressure can be taken for a specific fuel temperature. This prepressure must be such that the fuel just does not vaporize. The fuel vapor pressure characteristic line is dependent upon the type of fuel. Accordingly, for example, freshly tanked winter fuel already vaporizes at lower temperatures than corresponding summer fuel. In this way, a proper combustion of the fuel even under extreme cold winter weather is to be ensured. Correspondingly, the fuel vapor pressure characteristic line of winter fuel runs above the corresponding characteristic line of summer fuel. A safety reserve pressure is added to the value of the prepressure taken from the fuel vapor pressure characteristic line which safety reserve pressure is so selected that a vaporization of the fuel in the high-pressure pump is reliably avoided.
Alternatively, and in accordance with a further embodiment of the present invention, it is suggested that the prepressure be estimated based on a physical model of the high-pressure pump in dependence upon specific condition variables of the engine. As condition variables, the same condition variables are advantageously applied as for the model-based estimate of the fuel temperature.
As condition variables, especially the following are advantageously applied: the temperature of the engine, of the intake air and/or of the ambient temperature, the integral of the fuel throughput and/or of the air throughput, the pump capacity, the lost power and/or the efficiency of the high-pressure pump, the rpm of the high-pressure pump and/or of the engine, the fuel/air ratio lambda and/or the drive of the quantity or pressure control valve. The fuel temperature therefore does not have to be measured separately, but can be estimated based on specific condition variables of the engine which, as a rule, are anyway detected and are available.
Advantageously, the condition variables of the engine are weighted in dependence upon the kind of engine and on the operating point. According, for example, directly after the start of the engine, it can be necessary to weight the condition variables in such a manner that, for the modeled fuel temperature, the fact is accounted for that the fuel in the high-pressure pump directly after the start of the engine has a relatively low temperature independently of the condition variables of the engine and that this temperature slowly increases within increasing duration of operation of the engine.
According to a preferred embodiment of the present invention, it is suggested that the fuel vapor pressure characteristic line is determined for a worst-case scenario and is stored. A worst-case scenario is present, for example, for freshly tanked winter fuel. The vapor pressure curve of winter fuel requires relatively high prepressures. To this, a safety reserve is provided so that also in the worst-case scenario, a vaporization of fuel in the high-pressure pump is reliably avoided. If a fuel is present at low volatility (for example, summer fuel or old fuel), lower prepressures would be possible at the same temperatures than in the worst-case scenario. The spacing of the worst-case characteristic line plus safety reserve from the actual vapor pressure curve of the fuel present is unnecessarily great in this case.
According to another advantageous embodiment of the present invention, it is therefore suggested that the nature of the tanked fuel be detected and the stored fuel vapor pressure characteristic line be adapted to the type of tanked fuel. For detecting the type of tanked fuel, a tanking detection is utilized, which, for example, can distinguish summer fuel from winter fuel or fresh fuel from old fuel. Via the adaptation, the stored fuel vapor pressure characteristic line can be adapted to the actual fuel pressure vapor characteristic line and the safety reserve pressure can be reduced.
As a further solution of the present task, the invention suggests furthermore an arrangement of the type mentioned initially herein which has means for carrying out the method of one of the claims 1 to 10. The arrangement according to the invention can be configured as an independent control unit or can be integrated into a higher order control apparatus of the engine.
According to an advantageous further embodiment of the present invention, the high-pressure pump pumps fuel for a direct-injecting internal combustion engine. Especially for engines of this type, a formation of vapor in the high-pressure pump could occur at specific operating points which is now effectively prevented with the present invention.
According to another advantageous embodiment of the present invention, it is suggested that the low-pressure pump is configured as an electrical fuel pump. When, in accordance with the present invention, the prepressure with which the fuel is applied to the high-pressure pump can be reduced at specific operating points of the engine, a low-pressure pump configured as an electrical fuel pump has the advantage that it exhibits a reduced takeup of power, that is, consumes less current.